Abstract
Background:
In Part 1 of our study we demonstrated that simulated breathing effort (ie, inspiratory muscle pressure or ΔPmus) during AMV could be measured accurately by introducing an airway occlusion at end expiration to measure ΔPaw during the subsequent patient-triggered inspiration. In Part 3 of this bench study we examined Pmus measurements by EPM under varying conditions of chest mechanics. Our assumption was that under isovolumetric conditions Pmus should transmit to the ventilator circuit with little or no energy dissipation.
Methods:
Spontaneous breathing was simulated using an Ingmar ASL-5000 (IngMar, Pittsburgh, PA). Simulated chest mechanics were set as follows: Condition A: Resistance (R) and Compliance (C) 5 cm H2O per L/s and 80 mL/cm H2O; Condition B: R of 15 cm H2O per L/s and C: 80 mL/cm H2O; Condition C: 5 cm H2O per L/s and 30 mL/cm H2O. The breathing pattern was f of 20 at 3 ΔPmus levels reflecting mild, moderate and high effort (5, 10, 15 cm H2O) using a 20% pressure rise, 10% hold and 10% decay (inspiratory time or Ti: 1.2s). The test series consisted of 15 breaths/Pmus. A PB-980 ventilator (Medtronics, Minneapolis, MN) was used with continuous positive airway pressure (CPAP: 5 cm H2O). Trigger sensitivity was set at 3 L/m. Six clinicians measured Pmus using the ventilator's negative inspiratory force function during expiration and releasing the pause-hold as soon as a negative deflection in Paw was observed on the scalar waveform. An average of 10 breaths per observer was used for each condition. Multiple comparisons used one-way ANOVA and Tukey Kraemer post-test. Alpha was set at 0.05.
Results:
There were minor, statistically significant but clinically unimportant differences in Pmus measurements by EPM technique (Table).
Conclusions:
As would be anticipated, under conditions of no volume change, whereby energy is not dissipated in distorting the respiratory system, differences in chest mechanics have a negligible impact on Pmus measured during EPM. Therefore, EPM should accurately reflect Pmus regardless of pathological alterations in R or C.
†P< 0.001 vs. Condition A, ‡ P < 0.01 vs. Condition C, β P < 0.005 vs. Condition C View all access options for this article.
Δ Pmus (cmH2O)
Condition A (Low R, Normal C)
Condition B (High R, Normal C)
Condition C (Low R, Low C)
P (ANOVA)
5
4.0 ± 0.2
3.8 ± 0.4†‡
4.0 ± 0.2
<0.001
10
8.9 ± 0.4‡
8.1 ± 0.4 †‡
8.6 ± 0.7
<0.001
15
13.4 ± 0.6
12.3 ± 0.6†β
12.7 ± 0.5†
Get full access to this article